THE CATTLE GRUBS OR OX WARBLES FEN 
fore February 1 or after April 15. The maximum abundance usually 
occurs between February 20 and March 15. Im the plateau region 
of southwestern Texas in certain instances larve have been found to 
mature early enough in the fall to permit of the issuance of a con- 
siderable number of heel flies in the fall. It has not been possible 
to determine if this really takes place, but the finding of a young 
larva in the gullet of a cow on January 4 is fairly conclusive proof 
that some eggs are deposited in October or November. It is certain 
that some heel flies emerge and attack cattle during December, and 
heel-fly activity has been observed on numerous occasions in Jan- 
uary. The season of adult activity in New Mexico, Arizona, and 
California appears to be about the same as observed at Dallas. 
Throughout the central belt of States—that is, between the latitudes 
385 and 45° N.—the period of activity 1s approximately between 
March 15 and May 1, and in the northern tier of States between 
April 1 and June 30. ‘These dates are only approximate, and vary 
much during different years, as well as with altitude and local 
conditions. 
In New York adults of H. lineatum may begin oviposition as early 
as the middle of April, are most abundant during the first three 
weeks of May, and cease activity about the end of June. In the 
territory where //. bovis occurs it 1s difficult to determine the date of 
cessation of activity of the adults of H/. lineatum, since their seasons 
of activity overlap. 
In New York considerable numbers of adults of 7. bovis begin to 
oyiposit during the first week in June and apparently some emerge 
during the latter part of May. The flies are very annoying to cattle 
during June and the first half of July, are less so during the re- 
mainder of July and the first half of August, and during the re- 
mainder of August and up to the middle of September only strag- 
glers are abroad. Since normal larvee have been found in the backs 
of cattle in New York after September 1, some fly activity is possible 
throughout that month. No flies have been seen, however, nor have 
the effects of their attack been noted, after September 14. 
Hadwen (36) states that at Agassiz, British Columbia, adults of 
H. bovis appear in the early part of June and continue up to the be- 
ginning of August, and that adults of H. lineatwm were out from 
April 15 to April 24. Carpenter (79) records the emergence of 
adults of H. bowis in June and July, and deposition of eggs of ZH. 
lineatum on May 8 and June 16, at Athenry, Ireland. 
The season of oviposition of both species is about coincident with 
that of adult emergence. 
SEASONAL DEVELOPMENT OF LARVZE 
Rather abundant data regarding the seasonal occurrence of larve 
of 7. lineatwm in the gullets and viscera of cattle are presented on 
page 47 and Tables 3 and 4. In general, approximately two and one- 
half months elapse between the deposition of eggs and the earliest 
appearance of larve in the gullets. 
In the vicinity of Dallas, Tex., the larve first appear in the sub- 
mucosa of the gullets of cattle at about the end of March and con- 
tinue to enter that organ until the maximum is reached during Sep- 
